Zinc cyanamide has previously been produced by reacting calcium cyanamide (which contains approximately 1% CaS) with sulfuric acid or carbon dioxide and subsequently reacting the resultant aqueous solution of hydrogen cyanamide with a zinc oxide slurry at ambient temperatures such as 20.degree. C. for one to two hours. See, for example, Italian Patent No. 612539/1959 or British Patent No. 905957 (1962). This process is reported to produce a pigment grade product that contains 83%-85% zinc cyanamide. Unfortunately, the prior art processes require extensive washing in an effort to remove soluble inorganic salt contaminants. The products are generally not suitable corrosion inhibitors for mirror backing coatings because the soluble inorganic salt impurities, even when present in trace amounts, act as corrosion promoters. This is particularly true of soluble sulfide or sulfate contaminants present in protective paints and coatings used for protecting silver coated mirrors. Thus, although zinc cyanamide has been suggested as a corrosion preventive ingredient in mirror backing coatings (see for example U.S. Pat. No. 4,707,403 issued Nov. 17, 1987), it has never gained commercial acceptance for such applications. It now appears that because such available products hold considerable amounts of Ca.sup.2+ and So.sub.4.sup.2- as well as S.sup.2- containing soluble impurities, zinc cyanamides heretofore available have not been effective in mirror backing protective paint and coating formulations.
Zinc cyanamide functions as a corrosive preventive component of protective coatings and, more specifically, of mirror backing coatings due to its optimal water solubility and hydrolysis pH; however, it is reasonable to suppose that the product's H.sub.2 S scavenging capacity and specifically, the anti-oxidant nature of the cyanamide moiety, generated by solubilization during the pigment's interaction with corrosion promoting processes, are primarily accountable for its protective activity.
It is known that the reflective silver layer of mirrors (even protected by "backing" coatings) is extremely sensitive to the presence and corrosive action of the airborne salt particles (common in coastal regions) or acidic species and H.sub.2 S which are commonly present in industrial or domestic environments, respectively. Evidently, the above-mentioned various airborne contaminants promote oxidative processes on silver, which occur according to the scheme Ag.degree. Ag.sup.+ and result in the well known and undesirable phenomena, the corrosive destruction of mirrors' reflective layers. In this respect, it is interesting to note that chromates, zinc phosphate or zinc salts of dicarboxilic acids, commonly applied as a corrosion inhibiting additive to protective primers for steel or aluminum surfaces, contrary to all expectation, actually exhibit corrosive action on thin silver layers and attempts made to employ them in mirror backing applications were unsuccessful. It can be stated, that due to the above-mentioned sensitivity, mirrors' reflective silver layers are quite intolerant to quality variations of raw materials employed in mirror backing protective coatings. Such quality variations of pigment grade products are often caused by the presence of soluble inorganic contaminants, usually in trace amounts (i.e. water soluble by-products of the manufacturing procedures) or by low values of the products' specific surface area and, consequently, of their solubility rate typical for calcined grades.
There are also known manufacturing procedures in which zinc cyanamide is prepared by treating ZnO with excess urea or dicyandiamide, under N.sub.2 or vacuum, initially at 135.degree.-200.degree. C. and by calcining it subsequently at 600.degree.-800.degree. C. for two hours. The shortcoming of the above-mentioned energy intensive procedures is that they yield products of relatively "closed" texture, characterized by low value of specific surface area and porosity. Zinc cyanamide produced by calcination does not exhibit the highest possible degree of corrosion preventive activity necessary in mirror backing systems.